Border staff are ditching searches for illegal immigrants and drugs because of pressure to cut waiting times for travellers.

The damning revelations are exposed in a damning report set to be published today by Whitehall’s spending watchdog, the National Audit Office.

It reveals that crucial checks for illegal immigrants have been overlooked since the London Olympics amid pressure from ministers to cut waiting times at busy areas like Heathrow and Calais.

MPs said the report raised fears that illegal immigrants and human traffickers had been given open access to Britain for over a year.

Border Force officers told the National Audit Office that staff shortages had left them with little choice but to drop vital customs checks for illegal goods like guns and drugs - as well as illegal immigrants.

They blamed Government demands for full passport checks on everyone coming into the country alongside targets to slash queue times.

The pressure led to nearly 100% of passengers at the border receiving full passport checks in 2012-13, according to the NAO.

But Border Force staff told NAO investigators that they were not allowed to give as much attention to customs checks as they wanted - “especially during summer 2012 and at other peak times of the year.”

The report says: “In Calais, we observed officers being taken off controls to detect clandestine illegal entrants to the UK concealed in lorries in order to deal with passenger queues.”

“This occurred three times between 6 and 8 April 2013. In these three days, freight searching by Border Force officers was suspended on a further 19 occasions for reasons due to understaffing, such as allowing frontline officers to take meal breaks.”

The Audit Office said the number of people turned away at the border fell short of Government targets.

The report says: “Officials at two ports told us that when they were under pressure to reduce queues, they sometimes had to compromise on the amount of time they could spend questioning passengers who they suspected were unlikely to comply with their visa conditions.”

Yvette Cooper, Shadow Home Secretary, said the report had exposed the “chaotic and cavalier approach Theresa May has taken to border security and illegal immigration”.

She said: “The NAO found drops in checks for illegal migrants and criminal activity because of the Home Secretary’s irresponsible decision to cut 500 Border Force staff prior to the Olympics.

“We already raised concerns that illegal migrants caught on lorries in Calais were not being finger-printed. Now it is clear they often weren’t being checked at all.

“Cutting checks for clandestine illegal migrants is very serious. Tackling illegal immigration and human trafficking requires proper checks to be in place.

“Yet the Audit Office shows that patrols to prevent illegal migrants entering the UK were stopped three times in just two days. How many more times did it happen because of understaffing?”

Margaret Hodge MP, chair of the Committee of Public Accounts, said: “The Border Force did well to reduce queuing times both during and after the Olympics, but it is deeply worrying that this came at the expense of its other responsibilities, particularly customs.

“The Border Force must be able to check both goods and passengers at the same time - border security cannot be an either or choice.”

She added: “The Border Force must be in a position to deliver world-class border controls at all times.”

Amyas Morse, head of the NAO, added: “The Border Force now needs to show it can apply the lessons learned from its successes to date across its full range of activities to ensure the security of the border.

But Immigration Minister Mark Harper said: “The UK operates one of the most secure borders in the world.

“We inherited an organisation with significant challenges and, while some of these remain, I am confident that under the long term leadership of the new director general, Sir Charles Montgomery, Border Force will continue to build on its many areas of excellence.”